Retroviral-mediated gene transfer provides an efficient means of introducing exogenous genes into hemopoietic cells. By this method, we have shown that the naturally occurring Abelson murine leukemia virus containing the v-abl oncogene can infect hemopoietic progenitor cells and lead to generation of tumorigenic hemopoietic cell lines. Recombinant retroviral vectors containing the IL3 (interleukin 3) and GM-CSF (Granulocyte-macrophage colony stimulating factor) have also been constructed and recombinant viruses generated. Infection of established cell lines, but not primary hemopoietic cells, with the IL3 virus resulted in acquisition of tumorigenicity. We have also shown that hemopoietic stem cells can be infected with the IL3 virus and when they were infused into lethally irradiated mice, a disorder similar to a myeloproliferative syndrome in humans was detected in some of these animals.